In recent years, as the use of portable devices such as portable telephones, notebook type personal computers and the like have become popular, there is a strong need for the development of compact and lightweight secondary batteries that have a high-energy density. As such a battery, there are lithium ion secondary batteries that use lithium, lithium alloy, metallic oxide, or carbon as the anode.
Lithium composite oxide is used as cathode active material for the cathode material of a lithium ion secondary battery. Lithium cobalt composite oxide can be relatively easily synthesized, and imparts 4V class high voltage in a lithium ion secondary battery as the cathode material, so is expected to be used as material for making practical a secondary battery having a high-energy density. In regards to lithium cobalt composite oxide, research and development for achieving an excellent initial capacity characteristic and cycling characteristic in a secondary battery is advancing, and various results have already been obtained.
However, lithium cobalt composite oxide uses a cobalt compound, which is rare and expensive, as the raw material, so causes the cost of the cathode material and secondary battery to increase. A lithium ion secondary battery that uses lithium cobalt composite oxide has a unit cost per volume that is approximately 4 times that of a nickel-metal hydride battery, so its applicable uses are limited. Therefore, from the aspect of achieving portable devices that are more lightweight and compact, it is necessary to lower the cost of the cathode active material and to make possible the production of a less expensive lithium ion secondary battery.
One cathode active material that can be used in the place of a lithium cobalt composite oxide, is a lithium nickel composite oxide that uses nickel that is less expensive than cobalt. Lithium nickel composite oxide has the same high battery voltage as lithium cobalt composite oxide, has a lower electrochemical potential than lithium cobalt composite oxide, and decomposition due to oxidation of the electrolyte does not easily become a problem, so use as a cathode active material that will make possible higher capacity in a secondary battery is expected. In order for this, research and development of a lithium nickel composite oxide is also being actively performed.
Battery capacity is known to have a close relationship with the packing characteristic. The higher the packing density is, the more cathode active material can be packed inside an electrode having the same volume, so the battery capacity increases. When the particle density is the same, increasing the particle size is effective in order to increase the packing density. However, mixing of particles having a too large particle diameter, for example, particles that exceed 50 μm into the cathode active material, causes clogging of a filter during filtration after mixing of the cathode active material slurry, and also causes occurring of lineation (long thin shaped defects) during coating.
In regard to the packing characteristic of cathode active material, JP 2005-302507 (A) discloses a cathode active material that is constructed by a powder comprising secondary particles that are formed by an aggregation of primary particles and that have a spherical or elliptical shape having a particle size of 40 μm or less, with the percentage of particles having a particle size of 1 μm or less being 0.5% by volume to 7.0% by volume. With such a structure, it is possible to improve the packing characteristic, however, the particle size distribution is wide, so use as a cathode active material causes the voltage applied to the particles inside the electrode to become uneven, and minute particles selectively deteriorate due to repeated charging and discharging, which brings about a decrease in the battery capacity of the battery due to a decrease in the coulomb efficiency.
Therefore, for a cathode active material that is used in a battery that has high capacity and that can be industrially produced, it is necessary that the particle size be moderately large, and that the particle size distribution be sharp.
A lithium nickel composite oxide is obtained by calcination of a nickel composite hydroxide, which is the precursor to the lithium nickel composite oxide, together with a lithium compound. Particle characteristics such as particle size and particle size distribution of a lithium nickel composite oxide basically inherits the particle characteristics of the nickel composite hydroxide, so in order to obtain a cathode active material that has a desired particle size and particle size distribution, it is necessary to obtain a suitable particle size and particle size distribution in the precursor stage.
A crystallization reaction method is typically used as a production method for producing nickel composite hydroxide. In order to obtain particles having a sharp particle size distribution, a method of performing crystallization in a batch process is effective, however, a batch method has a disadvantage in that productivity is inferior compared to a continuous method that uses an overflow method. Furthermore, in order to obtain large particles in a batch process, it is necessary to increase the amount of raw material supplied, and as a result, the reaction vessel becomes larger, and productivity further decreases.
In order to improve productivity, a method is being investigated that suppresses an increase in amount of the reaction system by discharging solvent to the outside of the system while performing crystallization in a batch process. JP H08-119636 (A) discloses a method for generating nickel hydroxide particles in which nickel salt aqueous solution, ammonia water and alkali hydroxide aqueous solution are continuously supplied at fixed ratios, and before the reaction system overflows from the reaction vessel, solvent is removed to an extent that thorough mixing can be performed, and this increasing the amount of the reaction system by the supplying the feed solution for reaction and decreasing the amount by removing solvent is repeatedly performed. However, in this method, increasing the amount of the reaction system by supplying the feed solution for reaction and decreasing the capacity by removing solvent is repeatedly performed, so the number of particles per volume of solvent of the reaction system fluctuates and it is easy for particle growth to become unstable, so there is a problem in that particle size distribution worsens due to minute particles that are newly generated during the reaction.
Moreover, JP H07-165428 (A) discloses a method in which nickel salt aqueous solution, ammonia water and alkali hydroxide aqueous solution are simultaneously and continuously supplied at fixed ratios into a reaction tank that has a filtration function, and after the amount of the reaction system has reached a specified amount, the solvent in the reaction tank is continuously filtered by the filtration function and removed, and while keeping the amount of the reaction system nearly constant, nickel hydroxide is crystallized under specified mixing conditions. With this method, solvent is continuously removed by filtering, so the number of particles per volume of solvent in the reaction system becomes fixed, and the growth of particles is stable. However, with that example, the ratio of the supplied amount of raw material after removal of solvent has started with respect to the supplied amount of raw material up to when the removal of solvent is started is large. In that case, the growth rate from the particle size of particles generated in the initial stage of crystallization until the particle size in the final state of crystallization becomes large, and there is a possibility that minute particles will be generated in the reaction solution. Moreover, in the case where it is presumed that the number of particles does not change and the density is fixed, for example, in order to allow 2 μm particles to grow to 12 μm, it is necessary to increase the volume by 216 times. Therefore, in order to set a state in which a slurry concentration to which raw material has been added can be mixed until the particles grow to be 12 μm, it is necessary to reduce the initial slurry concentration. Therefore, the time during which raw material is supplied and caused to react becomes long, and productivity decreases. In this way, in the case of the method disclosed in JP 07-165428 (A), it can be said that producing particles having a large particle size is very difficult.
On the other hand, in JP H10-025117 (A) and JP 2010-536697 (A), in order to keep the slurry concentration in the system fixed, not only solvent, but also particles are discharged to the outside of the system, and the particles that were discharged to the outside of the system are returned to the reaction system. In the case that the particles that were discharged to the outside of the system are not returned to the reaction system, the number of particles in the reaction system decreases, and there is a possibility that the particle size will increase due to particle growth in this state. However, by returning the discharged particles to the reaction system, as in a continuous crystallization operation, minute particles that have not grown sufficiently, and coarse particles that have grown too much are mixed, so it is difficult to obtain particles having a sharp particle size distribution.